Plastics : Biodegradable versus compostable plastics

The fundamental difference:

Biodegradable plastics:

• Promises to break down naturally over time, but this is where the confusion begins. The term "biodegradable" has no standardized timeframe (it could mean 6 months or 50 years). It simply means microorganisms can eventually consume the material.

Compostable plastics:

• Must meet strict standards and they break down completely within 90-180 days in specific conditions, leave no toxic residue, and actually benefit the resulting compost by adding nutrients.

The making process:

Biodegradable plastics:

• PLA (Polylactic Acid): made from corn starch or sugarcane, but requires industrial composting facilities with temperatures of 140-160°F to break down properly. In home compost or landfills, it can persist for years.
• PHA (Polyhydroxyalkanoates): produced by bacteria, these plastics break down in marine environments and home compost piles. They are truly biodegradable in various conditions but are expensive to produce.
• Starch-based plastics: blend traditional plastic with starch, making them partially biodegradable. The starch portion breaks down, but plastic fragments often remain.

Compostable plastics:

• PBAT (Polybutylene Adipate Terephthalate): a fossil fuel-based plastic engineered to break down completely in industrial composting. Despite being petroleum-derived, it meets compostable standards.
• PLA with additives: standard PLA modified with accelerants to meet compostable certification requirements in industrial facilities.
• Bagasse-Based: made from sugarcane fiber, these break down in both industrial and some home composting systems.

Certification standards:

ASTM D6400 (US Standard):

• Must break down 90% within 180 days
• No toxic residue allowed
• Must support plant growth in resulting compost
• Requires testing at industrial composting temperatures

EN 13432 (European Standard):

• Similar to ASTM but with slightly different testing protocols
• More stringent requirements for heavy metal content
• Mandatory ecotoxicity testing

BPI certification:

• Third-party verification that products meet ASTM standards
• Allows use of official compostable logos
• Regular auditing of manufacturing processes

The disposal process:

Most certified compostable plastics need:

• Special industrial facilities
• Temperatures of 140-160°F sustained for weeks
• Controlled moisture levels (50-60%)
• Regular turning and aeration
• Specific carbon-to-nitrogen ratios

Carbon-to-nitrogen ratio: the proportion of carbon to nitrogen in organic material, expressed as a number like 30:1, microorganisms need both carbon (for energy) and nitrogen (for protein) to break down organic matter. 

Standard compostable plastics typically fail in backyard compost because:

• Home piles rarely maintain required temperatures
• Moisture and oxygen levels fluctuate
• Lack of professional monitoring
• Seasonal temperature variations

Both biodegradable and compostable plastics perform poorly in landfills because:

• Lack of oxygen prevents proper decomposition
• Compressed conditions inhibit microbial activity
• May produce methane (a potent greenhouse gas)
• Can persist almost as long as regular plastic

Environmental impact analysis: 

Carbon footprint:

• Biodegradable: often higher production emissions due to agricultural inputs (fertilizers, land use) but can be carbon-neutral if sourced sustainably.
• Compostable: similar production footprint but guaranteed end-of-life benefit through soil enhancement and carbon sequestration.

Resource competition:

• Biodegradable: large-scale production can compete with food crops for agricultural land, potentially affecting food prices.
• Compostable: same agricultural concerns, but shorter lifecycle means less long-term resource commitment.

Marine environment:

• Biodegradable: some types (like PHA) break down in ocean water, reducing marine pollution risk.
• Compostable: most require specific conditions not found in marine environments, so ocean disposal is still problematic.

Economic considerations:

Cost comparison:

• Regular plastic: $1.00 baseline
• Biodegradable plastic: $1.50-3.00 (150-300% more expensive)
• Compostable plastic: $2.00-4.00 (200-400% more expensive)

Infrastructure requirements:

• Biodegradable: may work with existing waste streams but performance is unpredictable.
• Compostable: requires significant investment in industrial composting infrastructure, which many regions lack.

Market scalability:

• Biodegradable: easier to scale production but inconsistent performance limits adoption.
• Compostable: more complex manufacturing but clearer value proposition for businesses and consumers.

Real-world performance issues:

Contamination Problems:

• Biodegradable: often contaminate recycling streams because consumers assume they're recyclable.
• Compostable: can contaminate both recycling and regular composting if not properly sorted.

Labeling confusion:

• Biodegradable: vague claims lead to "wishcycling" - consumers disposing incorrectly based on assumptions.
• Compostable: clear certification logos help, but many consumers don't understand the difference between home and industrial composting.

Seasonal Variations:

• Biodegradable: performance varies dramatically with temperature and humidity changes.
• Compostable: designed for controlled conditions, so performance is more predictable in proper facilities.

Best use cases:

Choose biodegradable when:

• Marine pollution risk is high (fishing gear, agricultural films)
• Industrial composting isn't available
• Lower cost is critical
• Performance requirements are basic

Choose compostable when:

• Industrial composting infrastructure exists
• Regulatory compliance requires certification
• Brand reputation depends on environmental claims
• Premium pricing is acceptable

Biodegradable plastics work best when you can't control disposal conditions but want some environmental benefit.

Compostable plastics are engineered solutions that are ideal when you have the infrastructure to support them properly.
The worst outcome is choosing based on marketing claims rather than actual disposal capabilities in your area.